An optical fiber is a thin glass fiber, whose diameter is about 150 micrometres. An optical fiber is used for replacing a copper conductor in telecommunications, because the data communication capacity of an optical fiber is much higher than that of a copper conductor. Furthermore, there is no shortage of the raw-material of the optical fiber, which is the case with copper.
Optical fiber is prepared in a so-called draw tower, which is a large tower with a height of about 8 to 9 metres, at whose top end molten glass is drawn downwards into a thin fiber. When the hot fiber travels downwards, it cools, whereafter, at the lower end of the tower, the fiber can be coated with a certain polymer in a particular coating vat. After coating, the fiber is transferred after a tension meter onto a particular roller, on which it is wound into a roll. It is known that bare fiber cannot be bent, because it breaks apart. After the polymer coating, the fiber endures bending very well. The draw rate of the fiber is at present about 4 to 10 metres per second and, among other things, the draw rate of the fiber and partly the thickness of the fiber are regulated by means of a tension meter. Currently, measurement of tension does, however, not take place until after the coating, because a bare fiber cannot be contacted. A tension measurement that takes place after the coating is, however, too late and provides an incorrect result concerning the tensile stress in a bare fiber. However, it would be desirable to increase the fiber draw rate, but in view of the process control, the tension of the bare fiber should be measured without contacting the fiber. The fiber tension meter that is used currently is a spring-loaded roller, over which a coated fiber advances. The magnitude of the loading indicates the fiber tension. There are some meters by whose means the tensile stress in a bare fiber can be measured by measuring the double refraction of light by directing light at the fiber and by measuring the magnitude of the double refraction. Such a meter is, however, highly complicated and expensive, and the phenomenon itself is weak. Therefore it has not gained popularity.
The object of the present invention is a novel acousto-optical meter, which measures the tension of a fiber by means of wire waves if the mass of the fiber per unit of length is known. By means of the meter it is possible to measure the tensile stress in the fiber without contacting the fiber and so that the advancing speed of the fiber is also compensated for. The principle is such that an advancing vibration is produced in the fiber, and the advancing speed v of said vibration is measured. If the mass of the fiber per unit of length is known (which is, as a rule, always the case), the tensile stress T in the fiber can be calculated from the simple formula: EQU T=v.sup.2 .multidot.M,
wherein M is the mass of the fiber per unit of length.
Thus, in the meter, first a suitable advancing wire-wave surge must be produced in the bare fiber, and the advancing speed of said surge is measured in a certain way without contacting the fiber. In such a case, the tensile stress can be calculated from the formula mentioned above.
As regards the prior art, the above electro-optical meter may be mentioned, which measures the tensile stress in the fiber on the basis of double refraction of light. The glass fiber material is often quartz, in which a double refraction of light proportional to the tension occurs. It is a problem in such a meter that the light has to be focused accurately on the moving glass fiber of a thickness of 150 micrometres, whereafter the double refraction has to be measured.
In the publications FI Patent 79,410 and U.S. Pat. No. 4,833,928, a solution has been described in which the tension in a moving thin film or membrane is measured in a way slightly similar to the present invention. A loudspeaker is brought near the membrane, by means of which loudspeaker a mechanical surge is brought about onto the membrane, which surge advances in the direction of the tension in the membrane. The advancing speed of the surge is measured by optically measuring the vibration of the membrane at two different points on the membrane. When the wave speed and the basis weight of the membrane are known, the tension can be calculated. However, such a meter cannot be used for measuring the tensile stress in a thin transparent glass fiber.
The publications U.S. Pat. No. 5,359,904 and FI 89,537 expressly describe a device for measurement of the tensile stress in an optical fiber by making use of a so-called wire wave advancing in the fiber and the measurement of the speed of said wave. However, in said publications, two separate optical measurement stations are employed, by whose means the speed of the wire wave is measured. Also, in said patent, reflections of the wire wave from the coating vat are not utilized either. The present invention, however, differs from this patent mentioned above in a number of essential different ways, as we shall see later. In the present invention, just one optical measurement station is employed for measurement of the tensile stress, and expressly the reflecting coming from the end of the fiber is utilized. Further, the compensation for speed is achieved automatically by means of this novel arrangement. The compensation for speed is becoming ever more important as the fiber draw speed becomes higher and during the "ramp-up" of the draw. Thus, the measurement device would be simplified to a considerable extent.